Pb-free solder alloy, and solder material and solder joint using same

ABSTRACT

A solder alloy based on an Sn—Zn—In—Ag system contains, in weight, 3.0%&lt;Zn&lt;5.0%, 0.1%&lt;In&lt;4.0%, 0.1%&lt;Ag&lt;0.4%, and the balance Sn. Therefore, the current Sn—Pb soldering method can be employed as it is. Further, a Pb-free solder material having a solder characteristic with excellent bonding strengths of the parts can be provided. Still further, since a difference between a solidus temperature and a liquidus temperature is small, floating of the parts leads can be suppressed, even in case where packaging processes are performed many times over. Still further, when the joint is exposed to the high temperature and high humidity atmosphere, the bonding strength can be prevented from being lowered.

FIELD OF THE INVENTION

The present invention relates to a Pb-free solder alloy, and a soldermaterial and a solder joint using same.

BACKGROUND OF THE INVENTION

Recently, the problem of the toxicity of lead (Pb) has invoked a strongmovement in regulating the disposal of lead to an environment. Thus, asa bonding material for parts of electronic products, a Pb-free solderhas been substituted for a conventional Sn—Pb solder.

As for characteristic properties of an alloy necessary as a soldermaterial, there are melting temperature, tensile strength, ductility (orelongation property), wettability, bonding strengths of parts joints,and the like.

A melting temperature of a solder is preferably to be set atapproximately 200° C. If a melting point of the solder is too high, itwill exceed heat resistance temperatures of the parts in a reflowsoldering, whereby a current soldering method may possibly incur damageson the parts. On the other hand, if a melting point of the solder is toolow, the solder becomes most likely melted such that the parts may falldown or be peeled off in case when the environmental temperaturesurrounding the parts becomes high.

As a solder for a reflow soldering, in which a lead is employed, thereis an Sn-37Pb solder alloy, typically. Alternatively, following Pb-freesolder alloys have been studied. For example, enumerated are Sn—Ag(—Cu)based, Sn—Cu(—Ni) based, Sn—Ag—Bi—Cu based, Sn—Zn(—Bi,—Al) based,Sn—In—Ag—Bi based solder alloys, and the like.

These are referred to as group I. Out of these, Sn—Ag(—Cu) based,Sn—Cu(—Ni) based, and Sn—Bi—Cu based solder alloys have alloycompositions whose melting points are measured in a range from 210° C.to 230° C. and are used for a flow soldering, a reflow soldering method,or the like. However, the melting points of these alloys are higher thanthat of a conventional Sn—Pb solder by 30° C. to 40° C. As a result,under a temperature condition of reflow soldering by using these alloys,the melting points thereof may exceed heat resistance temperatures ofthe parts. It is technically difficult to increase heat resistances ofcorresponding parts up to a temperature where reflow soldering can beperformed by using the aforementioned solder. Meanwhile, Sn—Zn(—Bi,—Al)based and Sn—In—Ag—Bi based solder alloys, and the like (referred to asgroup II), are employed in a field of PCB (printed circuit board)packaging in which a reflow soldering method is generally adopted.However, group II alloys are highly oxidized in a melting state in theair, and technically difficult to be applied in the flow solderingmethod at this point. While a group II alloy has many disadvantages as asolder in comparison with group I, it is advantageous in that itsmelting point can be adjusted to a temperature region close to that ofthe conventional Sn—Pb solder. Further, a group II alloy is used byadjusting composition thereof such that melting point thereof falls inthe range from approximately 180° C. to 210° C.

Namely, the Sn—Zn(—Bi,—Al) based solder alloy can be used under thecurrent reflow soldering condition since a melting point is in the rangefrom approximately 190° C. to 200° C. that is close to that of theconventional Sn-37Pb solder alloy, and is advantageous in being of lowcost among Pb-free solders. However, it has been considered thatwettability to a joint base material of a solder is not good. Further,it has been confirmed that bonding strengths of the parts aresignificantly deteriorated if the joint to be soldered with a Cu basematerial is exposed to a high temperature and a high humidity condition,even after the reflow soldering.

Further, it is likely that Zn is eluted from a solder into a flux,possibly incurring problems such as a lowered insulation resistance anda generation of migration, since Zn is employed in solder.

A melting point of the Sn—In—Ag—Bi based solder alloy is close to thatof an Sn—Pb solder, similarly to the case for the Sn—Zn based solder.When bonding this alloy system with the Cu base material, a Cu—Zncompound is not formed since Zn is not employed. Accordingly, such aphenomenon does not occur that a bonding strength in a bonding surfacewith Cu is significantly lowered under the high temperature and highhumidity atmosphere.

Meanwhile, in case of soldering to an Ag electrode, an Ag—In compound isformed in a bonding surface. It has been confirmed that the compound'sphase grows large as time passes and becomes fragile, whereby aninterface strength becomes lowered. In addition, it is observed that ifa heat cycle is applied in a state where the parts are bonded, a solderof a joint is deformed. A technological development for PCB has beendirected to a substrate designing of a narrower pitch, and a more highlevel packaging technology has been required.

Such a technical trend draws a concern that deformation of the soldermay cause short circuits. Further, since the solder contains a largeamount of rare and expensive Indium (In), the material cost amounts highand the continuous future supply may not be secured.

Solder alloys having melting points in the range from 180° C. to 210° C.are widely used in a soldering method in which soldering is performedseveral times (flow soldering after reflow soldering, reflow solderingafter reflow soldering, or the like), due to temperature characteristicsthereof. Here, the problematic point is that a place soldered once ispeeled off in subsequent soldering processes. Particularly, in alarge-scale IC parts or the like, parts leads float from a PCB, togetherwith solder. The reason for such a phenomenon is that on the secondsoldering or thereafter, a joint solder formed by a former soldering ispartially melted and a bonding strength thereof decreases, and, in sucha state, the joint is peeled off by a bending of a PCB or deformationsof the parts. Namely, in a solder alloy's property, the bigger adifference between a temperature where a solder alloy begins to melt(hereinafter, referred to as a solidus line) and a temperature where thesolder alloy completely melts (hereinafter, referred to as a liquidusline), the higher the possibility that the joint is peeled off.

In a conventional art, e.g., Japanese Patent No.2599890 (reference 1), amechanical strength or a creep resistance is improved by the addition ofZn to an Sn—Ag based solder.

At the same time, it is disclosed that a melting point becomes lower bythe addition of Zn or In.

However, the Ag concentration described in reference 1 is too high by asmuch as 1% in weight or more. For example, in an alloy of a high Agconcentration (1 weight %) such as Sn-6Zn-6In-1Ag, endothermic peakarea, whose summit is in the vicinity of a melting point of 200° C.,becomes large, as can be seen from the measurement results of DSC(differential scanning calorimetry) in FIG. 9. As a result, under areflow soldering condition same as that of the Sn—Pb solder, it islikely that the solder is not sufficiently melted down. If the solder isnot sufficiently melted down, fluidity of the solder is deteriorated,whereby a joint is not fully formed. In that case, voids in the solderremain to thereby lower the bonding strength. Further, in JapanesePatent Laid-Open Publication Heisei No. 9-174278 (reference 2), In isadded to an alloy of a near Sn—Zn eutectic composition so as to lower amelting temperature and improve wettability to parts metallization.Further, Ag is added so as to make Zn phase needle like solidificationmicrostructures in the Sn—Zn—In alloy into spheroidal solidificationmicrostructures and to finely disperse them. Therefore, the Znconcentration is set at from 6 to 11% in weight, and the Agconcentration is set at from 0.5 to 3% in weight.

The conventional Pb-free solder may incur various problems such as poorwettability due to Zn, which is a problem in the Sn—Zn(—Bi,—Al) basedsolder, and a lowered bonding strength with Cu electrode under thecondition of the high temperature and high humidity. Further, using raremetals such as In and Ag becomes a problem in the Sn—In—Ag—Bi basedsolder alloy.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to meet such acondition that a melting temperature characteristic is same as that ofan Sn—Pb based solder and to solve the problems of the conventionalSn—Zn(—Bi,—Al) based solder and the Sn—in—Ag—Bi based solder.

Particularly, it is an important object to improve solder jointreliability under the condition of the high temperature and highhumidity.

For achieving the objects, a solder alloy in accordance with the presentinvention is based on an Sn—Zn—In—Ag system having, in weight,0.3%<Zn<5.0%, 0.1%<In<4.0%, 0.1%<Ag<0.4%, and the balance Sn.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph for showing peel strengths of solder joints of soldersin accordance with Example 1 of the present invention, as a function ofexposure time;

FIGS. 2A to 2E show DSC measurement results of solder alloys as afunction of temperature, in case when Zn is added to Sn-3In-0.3Ag ofExample 1 of the present invention while varying the Zn concentration inthe range from 2 to 6 weight %;

FIGS. 3A to 3C describe typical views of structures, in case when asmall amount of Ag is added to Sn-4Zn-3In of Example 2 of the presentinvention;

FIG. 4 illustrates a graph for showing electrochemical corrosionpotentials as a function of time, in case when a small amount of Ag isadded to Sn-4Zn-3In of Example 3 of the present invention;

FIGS. 5A to 5E explain variations of melting temperatures as a functionof Ag concentration, when a small amount of Ag is added to Sn-4Zn-3In ofExample 1 of the present invention;

FIG. 6 offers a graph for showing variations of mechanical properties ofsolder alloys as a function of In concentration, in case when In isadded to Sn-4Zn-0.3Ag of Example 6 of the present invention in a rangefrom 0 to 10 weight %;

FIG. 7 sets forth to a graph for showing variations of mechanicalproperties of solder alloys in accordance with Example 8 of the presentinvention, as a function of exposure time;

FIG. 8 presents a graph for showing variations of mechanical propertiesof another solder alloys in Example 8 of the present invention, as afunction of exposure time; and

FIG. 9 depicts a DSC measurement result of a conventional Sn-6Zn-6In-1Agalloy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to accompanying drawings.

In accordance with the present invention, a solder alloy is anSn—Zn—In—Ag solder containing a small amount of Ag for preventing abonding interface strength from being lowered when a joint of a Cu basematerial with a solder is exposed to the high temperature and highhumidity atmosphere, based on an Sn—Zn—In based solder having a meltingpoint of 210° C. or less.

In aspects of the melting temperature and bonding reliability, it ispreferable that the concentration of each element in such a solder alloyis as follows, in weight:

-   -   3.0%<Zn<5.0%;    -   0.1%≦In<20.0%; and    -   0.1%≦Ag≦0.4%.

Hereinafter, the composition range will be explained.

The Zn concentration is from about 3.0% to 5.0% in weight. When the Znconcentration is less than 3.0% in weight, a melting point of the soldercannot be lowered to about 200° C. Further, if the Zn concentration isless than 3.0% in weight, a difference between a solidus temperature anda liquidus temperature becomes large even though the In concentration isincreased. As a result, in multiple soldering processes, it is likelythat the parts joints are peeled off.

On the other hand, when the Zn contention is more than 5.0% in weight, abonding interface strength with a Cu film is lowered under the conditionof the high temperature and high humidity. Further, if the Znconcentration increases, wettability of the solder becomes deteriorated,resulting in oxidation of the solder and a lowered electrical insulationof the joint.

The In concentration is from 0.1 to about 20.0% in weight. When theconcentration is less than about 0.1% in weight, a melting point cannotbe expected to be lowered. If the In concentration is more than 20.0% inweight, a solidus temperature in the solder melting point becomes toolow. In case of the Sn-20In, a solidus temperature is 153° C. If thesolidus temperature decreases, the solder is melted and peeled off whenbeing exposed to a high temperature environment.

Further, the same failure may possibly be caused due to heat generationby using equipment. Still further, since the solidus temperature (153°C.) and the liquidus temperature (199° C.) of the Sn-20In are separatedfrom each other too far, such a phenomenon may occur that the solder ispeeled off in the second soldering process or thereafter.

The Ag concentration is between 0.1% and 0.4% in weight. If theconcentration is less than about 0.1% in weight, an effect that preventsa bonding strength from being lowered cannot be obtained when exposingto an environment of high temperature and high humidity after soldering.

If the Ag concentration exceeds 0.4% in weight, the solder tends to meltat a higher temperature in a melting point temperature area of thesolder, so that fluidity of a molten solder becomes poor in the reflowsoldering process.

Further, it is more preferable that the composition range is as followsbelow, in weight:

-   -   0.3%<Zn<5.0%;    -   0.1%≦In≦4.0%; and    -   0.1%≦Ag≦0.4%.

If the In concentration in the solder alloy increases, ductility of thesolder alloy becomes deteriorated. Further, if the In concentration is4% in weight and less, elongation of 30% or greater can be assured.Therefore, a stress can be relieved since the solder is deformed when astress due to heat-shock or the like is on. In contrast, if the solderdoes not have ductility, crack may possibly be developed in the solderjoint in case where a PCB or parts are expanded or shrunk.

Meanwhile, ‘high temperature and high humidity’ of the present inventionmeans a circumstance of 85° C. and 85% RH (relative humidity).

EXAMPLES Example 1

In Example 1, a peel strength of a joint having, in weight, 3% In and0-6% Zn (the remaining portion was Sn) was measured, with respect to avariation of a bonding strength when exposed to an environment of thehigh temperature and high humidity.

First, a solder alloy, which was mixed to have a predeterminedcomposition, of about 1 kg was held at 230° C. And then, QFP (Quad FlatPackage) parts of 0.65 mm in pitch and 100 pins are fixed to aCu-attached glass epoxy PCB by using an adhesive. This specimen wasapplied to a flux, and then, subjected to soldering by dipping into thesolder. A soldered article was washed with acetone by using a microwavewashing machine, so that residuals of the flux were removed. A solderedPCB specimen after being washed was put into a hygro-thermostat(constant temperature and humidity oven) kept at 85° C. and 85% RH, andthen, a peeling strength of a lead bonding strength was measured forevery 250 hours.

FIG. 1 shows a variation of a lead bonding strength, in case whensoldering QFP parts with a solder having, in weight, 3% In, 0-6% Zn, andthe balance Sn. Here, 0-6% Zn means that the Zn concentration is in therange from 0 to 6% in weight. Further, it can be noted that as the Znconcentration increases, a bonding strength when being exposed to acondition of high temperature and high humidity significantly declines.Further, in case where the Zn concentration is 6% in weight, a bondingstrength at exposure time of 500 hours becomes 1 kgf or less.

Namely, as the Zn concentration in the solder increases, bondingstrengths of the parts tend to decrease under an environment of the hightemperature and high humidity. Zn phases in the solder diffuse into abonding surface and react with a Cu base material under the hightemperature and high humidity atmosphere, to thereby form and grow aCu—Zn compound layer. In the course of the process, Zn oxidizes due toan effect of high humidity, whereby a bonding strength in an interfaceof the Cu—Zn compound layer of the bonding surface with the solder issignificantly lowered. As can be seen from FIG. 1, it is preferable thatthe Zn concentration is less than about 5% in weight.

Meanwhile, FIGS. 2A to 2E describe DSC measurement results of solders,each having, in weight, 3% In, 2-6% Zn, 0.3% Ag, and the balance Sn. Ifthe Zn concentration is less than 3% in weight, a melting point of ametal exceeds 210° C. Therefore, it is preferable that the Znconcentration is greater than about 3% in weight.

Further, if the Zn concentration is greater than 5% in weight, a bondingstrength under the high temperature and high humidity is graduallylowered. Thus, it is preferable that the Zn concentration is less thanabout 5% in weight.

Example 2

Example 2 was carried out for observing a structure, in case when asmall amount of Ag is added to an Sn-4Zn-3In. Each solder having, inweight, 4% Zn, 3% In, 0.1-0.5% Ag, and the balance Sn of about 0.6 g wasmelted on a ceramic plate to form a sphere shape, and in that state,cooled in the air. A section of each solder particle was polished andobserved by using scanning electron microscope (SEM). The results weredescribed in FIGS. 3A to 3C.

As is known from FIGS. 3A to 3C, needle like Zn phases decrease as theAg concentration increases. Further, it can be noted that spheroidalZn—Ag phases increase in FIGS. 3B and 3C. Still further, a finestructure of the solder is confirmed. Zn phases are finely dispersed, sothat a connection between the Zn phases disappears. Accordingly,oxidation of Zn, which causes to lower a bonding strength, does notspread towards an inside of the solder, and lowering of a bondingstrength under the condition of the high temperature and high humidityis suppressed.

In case where the Ag concentration is 0.1% in weight, many needle likeZn phases are observed, as shown in FIG. 3A. However, a spheroidal Zn—Aglayer is certainly confirmed.

Example 3

In Example 3, a variation of an electrochemical corrosion potentialwould be explained, in case when a small amount of Ag is added to theSn-4Zn-3In.

Each solder having, in weight, 4% Zn, 3% In, 0-0.5% Ag, and the balanceSn was prepared with a bar shape having a cross section of 5 mm×5 mm. Asurface of the bar-shaped specimen was polished with water resistancepolishing paper of 1200 mesh, and then, subjected to buffing by usingAl₂O₃ suspension. Subsequently, the specimen was immersed into a 3.5 wt.% NaCl water solution at 25° C. Further, by using a standard electrodeemploying a silver electrode, a silver chloride electrode, and asaturated KCl water solution, an electromotive force difference, whichis generated between Ag of the standard electrode and the solderspecimen, was measured. The result was shown in FIG. 4. Still further,as a reference sample, an electrochemical corrosion potential of theSn-3In solder not containing Zn was described.

As is known from FIG. 4, oxidation of Zn in the solder becomes difficultas an electromotive force is close to that of the Sn-3In solder. Namely,by the addition of Ag with 0.1% in weight or more, such an effect can beobtained that oxidation is prevented from progressing.

Example 4

In Example 4, an observation result of a bonding surface would beexplained when soldering the Sn-4Zn-3In—0.3Ag with a Cu plate. TheSn-4Zn-3In-0.3Ag solder of 0.3 g was placed on the Cu plate and appliedto a flux. Then, it was heated on a 230° C. heat plate and soldered.After this specimen was filled into a resin, polished, and evaporated, asection of the bonding surface was observed by using SEM and X-ray microanalyzer (XMA). As a result of the observation using SEM and XMA, a Znlayer and an Ag layer could be observed to be generated in a bondingsurface between the solder and the Cu plate. Namely, it can be knownthat a Zn—Ag phase is formed in the bonding surface between the Cu plateand the bonding surface. If a Zn—Cu compound phase is formed in abonding surface, oxidation in an interface of the solder with the Zn—Cucompound progresses, so that a bonding strength is lowered. Namely, bypreventing the formation of the Zn—Cu compound layer, a bonding strengthcan be prevented from being lowered.

Example 5

In Example 5, a variation of a melting point would be explained, in casewhen a small amount of Ag is added to the Sn-4Zn-3In. FIGS. 5A to 5Eshow measurement results of melting points of solders, each having, inweight, 4% Zn, 3% In, 0-0.5% Ag, and the balance Sn, by using DSC. Ascan be seen from FIGS. 5A to 5E, it could be noted that as the Agconcentration increases, a peak representing a heat absorbing amount inthe vicinity of 205° C. to 210° C. becomes large, and a melting amountof the solder increases in this temperature area. If the Agconcentration becomes 0.5% in weight, an endothermic peak in thevicinity of 205° C. to 210° C. grows as much as substantially same asthat in the vicinity of 190° C. As a result, in case when being employedas a solder, it is difficult to be melted. In other words, the solder ismelted at a lower temperature (about 193° C.) first, and melted again ata higher temperature. Further, wettability or fluidity of the solderbecomes deteriorated.

From the aforementioned measurement results, by an addition of Ag with0.1% in weight or more, an electrochemical corrosion potential isimproved. On the other hand, if Ag is added more than 0.5% in weight, asdescribed by DSC measurement of the alloy, higher temperature peaksincrease. Accordingly, the solder is difficult to be melted, so thatwettability or fluidity characteristic thereof becomes deteriorated.

Further, if Ag is added to the solder containing Zn, needle like Znphases decrease and spheroidal Zn—Ag phases increase. As a result, afine structure of the solder can be confirmed by a structureobservation. While needle like Zn phases are observed in case where theAg concentration is 0.1% in weight, an effect of improving anelectrochemical corrosion potential can be obtained even in that case,as mentioned above.

Further, by the addition of Ag, the Zn—Ag compound phase is formed in abonding surface when soldering on a Cu, to thereby serve as a barrierlayer for suppressing a reaction between Cu and Zn. As a result,formation of the Zn—Cu compound layer, which tends to be easilyoxidized, can be suspended, so that oxidation in the bonding surface issuppressed to thereby prevent the bonding strength from being lowered.

Example 6

Each solder having, in weight, 4% Zn, 0-10% In, 0.3% Ag, and the balanceSn was molded to a plate shape at a temperature higher than a solderliquidus temperature by 50° C., and a tensile specimen was prepared.

The specimen was JIS4 specimen. The tensile test was performed at atensile rate of 5.0 mm/min.

The result was described in FIG. 6. As is evident from FIG. 6,elongation of 30% or greater is held in the range from 0 to 4% In, inweight.

Example 7

Preferably, a Pb-free solder material formed of a solder alloy and aflux is utilized in a wire solder and a cream solder. Here, the solderalloy is based on the Sn—Zn—In—Ag system having, in weight:

-   -   3.0%<Zn<5.0%;    -   0.1%≦In≦4.0%;    -   0.1%≦Ag≦0.4%; and

the balance Sn.

Further, as the flux, a known flux may be used.

Example 8

In Example 8, a solder bonding strength would be explained by using asolder alloy, which has at least one element selected from a groupconsisting of Ni, Ti, Mg, Al, and Co, based on the Sn—Zn—In—Ag systemhaving, in weight:

-   -   3.0%<Zn<5.0%;    -   0.1%≦In≦4.0%; and    -   0.1%≦Ag≦0.4%.

Here, a total concentration of at least one element is in the range from0.001% to 0.05% in weight and the remainder is Sn.

The high temperature and high humidity test was carried out on thefollowing samples. FIG. 7 exhibits variations of bonding strengthsthereof. Bonding strength was measured by the same method as inExample 1. The samples were prepared by using solder alloys, each havingone of the aforementioned elements and performing a reflow soldering ona Cu film.

In FIG. 7, F represents a standard Pb-free solder alloy of the presentinvention. Further, A, B, C, D, and E have the same composition with Fother than Sn, and contain 0.004% Ti, 0.01% Ni, 0.01% Mg, 0.05% Al, and0.05% Co, in weight, respectively. And, the remaining portion thereof isSn. Comparing bonding strengths after being exposed for 1000 hours underthe condition of the high temperature and high humidity, the samples A,B, C, and E are found to be superior to the standard F. Further, it canbe noted that the sample D maintains a bonding strength at least equalto or greater than F.

FIG. 8 shows variations of bonding strengths under the high temperatureand high humidity on three solder joint compositions: Sn-8Znn-3Bi,Sn-4Zn-3In-0.3Ag, and Sn-4Zn-3In—0.3Ag-0.003Ti. Further, the solderjoints are formed by the same manner as in Example 1. As can be seenfrom FIG. 8, the addition of Ti is clearly demonstrated to be effectiveafter 1500 hours.

Further, in the comparative Sn-8Zn-3Bi, a bonding strength becomes lessthan 1 kgf after 250 hours. Other elements such as Ni, Mg, Al, and Coprovide the same effects as Ti.

Example 9

A Pb-free solder material formed of a solder alloy and a flux of Example9 is utilized in a wire solder and a cream solder. Here, the solderalloy has at least one element selected from the group consisting of Ni,Ti, Mg, Al and Co, based on the Sn—Zn—In—Ag having, in weight:

-   -   3.0%<Zn<5.0%;    -   0.1%≦In≦4.0%; and    -   0.1%≦Ag≦0.4%.

Here, a total concentration of at least one element is in the range fromabout 0.001% to about 0.05% in weight and the remainder is Sn.

Further, as the flux, a known flux may be used.

As mentioned above, in accordance with the present invention, the Znconcentration is in the range from about 3 to 5% in weight, so thatsolder joint reliability can be improved under the high temperature andhigh humidity atmosphere. Further, a solder alloy of the presentinvention may be a bar solder (molten solder), and a Pb-free solderalloy suitable for a diffusion bonding. Still further, the presentinvention may include a solder joint of electrical and electronicequipment using the solder alloy of the present invention.

A Pb-free solder using a solder alloy in accordance with the presentinvention has a melting temperature substantially equal to that of aconventional Sn—Pb solder. Therefore, the current Sn—Pb soldering methodand the current parts or production equipment can be employed as it is.Further, a Pb-free solder material having a solder characteristic withexcellent bonding strengths of the parts can be provided.

Further, since a difference between a solidus temperature and a liquidustemperature is small, floating of the parts leads can be suppressed,even in case where packaging processes are performed many times over.Still further, when the joint is exposed to the high temperature andhigh humidity atmosphere, the bonding strength can be prevented frombeing lowered.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A solder alloy based on an Sn—Zn—In—Ag system, the solder alloycomprising, in weight: 3.0%<Zn<5.0%; 0.1%≦In≦4.0%; 0.1%≦Ag≦0.4%; and thebalance Sn.
 2. A Pb-free solder material comprising a solder alloy and aflux, wherein the solder alloy is based on an Sn—Zn—In—Ag system having,in weight: 3.0%<Zn<5.0%; 0.1%≦In≦4.0%; 0.1%≦Ag≦0.4%; and the balance Sn.3. A solder alloy based on an Sn—Zn—In—Ag system and having at least oneelement selected from the group consisting of Ni, Ti, Mg, Al, and Co,the solder alloy comprising, in weight: 3.0%<Zn<5.0%; 0.1%≦In≦4.0%; and0.1%≦Ag≦0.4%, wherein a total concentration of said at least one elementis in the range from about 0.001% to about 0.05% in weight, and theremaining portion thereof is Sn.
 4. A Pb-free solder material comprisinga solder alloy and a flux, wherein the solder alloy has at least oneelement selected from a group consisting of Ni, Ti, Mg, Al, and Co,based on an Sn—Zn—In—Ag based solder alloy having: 3.0%<Zn<5.0%;0.1%≦In≦4.0%; and 0.1%≦Ag≦0.4%, wherein a total concentration of said atleast one element is in the range from about 0.001 to about 0.05% inweight and the remaining portion thereof is Sn.
 5. A solder joint ofelectrical and electronic equipment comprising the solder alloy ofclaim
 1. 6. A solder joint of electrical and electronic equipmentcomprising the solder alloy of claim 3.